Flat UI was built on top of Twitter's well crafted Bootstrap interface toolkit and allows developers to quickly put user interfaces together by grabbing and assembling the parts they want from Flat UI.

It uses Google Web Fonts, many CSS 3 effects and was written in an OOCSS coding style.

It also comes with Retina support out of the box and it's easy to customize with different color schemes.

Flat UI is also available under a commercial license, as the Flat UI Pro package, about which you can read more details here. This provides access to the Flat UI PSD and extra HTML widgets.